1. Field of the Invention
The invention relates to a salt-free process for preparing cyclobutanone by isomerizing cyclopropyl-methanol (cyclopropylcarbinol) to cyclobutanol over a heterogeneous acidic catalyst and subsequently catalytically dehydrogenating it to cyclobutanone and the cyclobutanone derived therefrom.
2. Description of the Background
Cyclobutanone is an important and interesting material in organic chemistry, finding use in active ingredients synthesis, for example active pharmaceutical ingredients.
The preparation of cyclobutanone by conversion of cyclopropylmethanol to cyclobutanol and subsequent oxidation is a synthetic route generally known in the literature (Org. Synth. 1981, 60, 20–25).
The isomerization of cyclopropylmethanol to cyclobutanol in the presence of dilute hydrochloric acid is described, for example, in Org. Synth. 1986, 64, 50–56. Only in high dilution (cyclopropylmethanol dilute hydrochloric acid weight ratio=1:12) does the process deliver the desired cyclobutanol. For workup, all of the acid is neutralized and the aqueous phase is additionally saturated with sodium chloride and only then extracted with diethyl ether (yield 57%). From a technical point of view, this process is of little advantage as a consequence of the high salt burden and also the low space-time yield. Furthermore, the use of hydrochloric acid results in chlorinated by-products which are virtually impossible to remove by distillation (Can. J. Chem. 1980, 58(11), 1075–1079) and lead to corrosion and stability problems in the subsequent oxidation step. When using hydrochloric acid of higher concentration, the proportion of chlorinated by-products increases. For example, U.S. Pat. No. 5,905,176 describes the preparation of cyclobutyl chloride by reacting cyclopropylmethanol with 36% hydrochloric acid in the temperature range of 35–120° C.
The processes so far described for the oxidation of cyclobutanol to cyclobutanone use customary oxidizing agents in organic synthesis. A comparison of the existing oxidation methods is given in DE 199 10 464. In general, these are oxidations carried out in homogeneous systems from which the desired cyclobutanone first has to be isolated by extraction or the like. These processes generally lead to large amounts of waste and also to high burdens of toxic reagents, for example when using CrO3/oxalic acid according to Org. Synth. 1981, 60, 20–25. In this process, the direct oxidation of the solution of cyclobutanol in aqueous hydrochloric acid obtained from the isomerization of cyclopropylmethanol is described. For the conversion of 49.5 g of cyclopropylmethanol (results in 14–16 g of cyclobutanone, yield 31–35%), approximately 1.2 l of strongly acidic, chromium-containing wastewater is formed.
DE 199 10 464 likewise describes a one-stage batchwise process for preparing cyclobutanone starting from cyclopropylmethanol by initially isomerizing the cyclopropylmethanol with dilute hydrochloric acid and then oxidizing the aqueous solution obtained with sodium hypochlorite solution. As a consequence of the large amounts of dilute hydrochloric acid and sodium hypochlorite solution required, the process likewise leads to a considerable salt and wastewater burden (according to Example 2, approximately 1,600 g of wastewater for 60 g of cyclobutanone). Moreover, the performance of the oxidation using sodium hypochlorite solution in a strongly acidic medium leads to an increased formation of chlorine which escapes from the reaction mixture in gaseous form. The associated safety and corrosion problems and also the increased formation of chlorinated by-products makes the process relatively unattractive from a technical viewpoint. In addition, storage tests of the cyclobutanone prepared according to the German patent application DE 199 10 464 lead to significant decomposition. Strong coloration of the material is observed within a short time, especially in batches having a high chlorine content.